In order to provide high-speed data transmission, optical communication systems have become widespread. In order to achieve increased speed and/or longer distance transmission in the optical communication system, it is important to develop an optical transmitter which generates and transmits an optical signal.
FIG. 1 illustrates an example of an optical transmitter. The optical transmitter 1 includes a modulator driver 2, a light source (LD) 3, and a modulator 4. A data signal is input to the optical transmitter 1 through a signal input terminal. The modulator driver 2 generates a drive signal from the input data signal. At this time, the modulator driver 2 generates the drive signal according to a control signal and/or control voltage. The light source 3 is a direct-current laser light source, and generates CW (Continuous Wave) light. The modulator 4 modulates the CW light with the drive signal generated by the modulator driver 2 to generate a modulated optical signal.
In order to achieve the long distance transmission of an optical communication system, the output optical signal waveform of the optical transmitter 1 is appropriately shaped. For example, the control voltage and/or control signal given to the modulator driver 2 is set so as to satisfy the demand for an output optical signal waveform such as a crosspoint, an extinction ratio, or a pulse mask.
However, the output optical signal waveform is largely dependent on the waveform of the input signal to the optical transmitter 1. For example, when the amplitude of the input data signal to the optical transmitter 1 fluctuates, it is not easy to provide sufficient tolerance for fluctuations so as to satisfy the demand for the output optical signal waveform. For this reason, the optical transmitter 1 may include an interface circuit 5 between the signal input terminal and the modulator driver 2 to compensate for the fluctuations in the amplitude of the input signal, as necessary.
As a technique related to the present application, an optical transmitter has been proposed that is capable of obtaining, in the optical output, an optimal amount of fluctuation in the optical frequency without causing a deterioration of the optical output power or the output optical signal waveform. This optical transmitter includes a light source, an electroabsorption optical modulator (EA modulator) which absorbs the input light according to the applied voltage, an optical modulator driver, a bias circuit, an optical coupler which branches the optical output, an optical frequency fluctuation monitor having an optical frequency discriminator and a peak detector, and a controller. The drive condition of the optical modulator is adjusted such that the amount of fluctuations in optical frequency will be optimized (for example, see Japanese Laid-open Patent Publication No. 11-305175).
Moreover, as another related technique, the following optical modulator driver has been proposed. The optical modulator driver has resistors respectively at the sources of two transistors which configure a differential circuit with first and second input ports. At least one of the resistors is variable, and different values of resistance are set to a pair of differentials. The optical modulator is driven in the state where the crosspoint of eye patterns is shifted away from the center (for example, see Japanese Laid-open Patent Publication No. 11-014951).
Further, in Japanese Laid-open Patent Publication No. 2009-168833 and Japanese Laid-open Patent Publication No. 2004-061556, other related techniques are disclosed.
In recent years, there has been an increasing demand for the reduction in size and power consumption in optical modules (including optical transmitters). For this reason, in the optical transmitter 1 of FIG. 1, a configuration in which the interface circuit 5 is not implemented is desired. However, in the configuration in which the interface circuit 5 is not implemented, as described above, the output optical signal waveform may deteriorate when the amplitude of the input data signal fluctuates.